Testing of Evaluated Transactinium Isotope Neutron Data and Remaining Data Requirements H. Kiisters Nuclear Research Center Karlsruhe Institute of Neutron Physics and Nuclear Engineering 1. Introduction The first international meeting on transactinium isotope nuclear data, held in Karlsruhe in 1975 /l/, mainly dealt with neutron cross-sections and other nuclear properties as e.g. decay constants of minor actinide isotopes, abbreviated in this paper to MINACs. For the plutonium and the thorium fuel cycle, the MINAC isotopes are 230Th up to 244C, with the exception of the main nuclei 232Th, 233U, 235U, 23aU, 23gPu, 240Pu and 241Pu (sometimes the higher Pu isotopes are incorrectly counted as MINACs; for special interests the curium isotopes beyond mass number 244 are also included in the MINAC list). Minor actinide isotopes are understood to be of "minor importance" in thermal and fast reactor design. This minor importance is regarded with respect to the influence of these nuclei on the reactivity balance or the neutron spectrum of a nuclear plant. However, the MINACs can be of prime importance in the out-of-pile stages of the nuclear fuel cycle, especially in determini% the decay heat and the neutron as well as alpha, beta and gamma radiation of spent nuclear fuel after discharge from the reactor. These aspects are essential for designing safe transport casks and inter- mediate storage facilities, reprocessing units and waste disposal sites. The intensive discussion on the nuclear data aspects of MINACs originated from the predicted expansion of nuclear energy after the first oil crisis in 1973. Reprocessing units of sizes up to 1500 to/yr throughput were dis- cussed, recycling of Pu in thermal reactors was, and in some countries is considered as a serious strategy besides the introduction of Pu into fast ~breeder reactors; also U-recycling came into the discussion. Almost in 92120001 2 . J parallel the problem of safeguarding fissile material became essential, leading to "dirty fuel cycle concepts" in the INFCE discussions. Here minor actinides (mainly those of the thorium cycle) played a major role. These ambitious plans meanwhile have changed and become more moderate because the nuclear energy demand has not increased as anticipated; repro- cessing plants are designed now for a size of about 350 to/yr throughput and alternate fuel cycles are practically out of discussion; direct storage of spent fuel has gained some interest (for economic reasons). Presently extended burn-ups of PWRs up to and even above 50000 MWd/t are under investigation besides recycling of Pu in thermal reactors. These more "down to earth" projects nevertheless all require a good know- ledge of the concentrations of minor actinides in spent fuel, because the 0 concentrations of these isotopes increases both with extending the burn-up and with multiple recycling of fissile material in thermal reactors. Some studies continue to investigate actinide recycling (Am, Cm) to reduce the hazards in a perpetual storage from the long-lived alpha emitters besides plutonium, or to separate Np from the waste. Also studies on re- cycling actinides and non-volatile fission products using the AIROX pro- cess are performed. To determine the possible risks associated with fuel handling in the various stages of these complex fuel cycles, the radiation level of the MINACs is of prime importance, strongly depending on the corresponding nuclear data in the generation and decay process. Having observed in 1975 large discrepancies and gaps for the MINAC cross- sections in the data files existing at that time, a fruitful international effort started to improve the data basis of MINACs, both experimentally and theoretically. To test the adequacy of these improved data, C. Reich requested in 1979 121 "well-documented benchmark-quality depletion measure- ments or to identify if they exist". Although already the second Advisory Group Meeting in 1979 /3/ included also the data status of the main transactinium isotopes, the present paper will summarize the work of testing the minor actinide neutron data only; the status of the main transactinium isotope nuclear data is well known from the experience in thermal and fast reactors; important long standing 92120002 discrepancies as e.g. the neutron capture data for 2381J could not be re- moved up to now, and there seems to be little hope that this is possible with the present experimental techniques. These data will not be discussed here. In this paper, experimental and calculated resonance integrals as well as measured and calculated nuclide concentrations in post-irradiation analy- ses only for MINAC isotopes are compared. The paper will be restricted to the U-Pu fuel cycles in thermal and fast reactors. From this intercomparison and from sensitivity studies, remaining data requests are deduced. 2. Formation of Minor Actinides in Light Water Reactors and Fast Reactors, and Related Nuclear Data Requests As already mentioned in the introduction, the discussion in this paper deals with nuclides in the U-Pu fuel cycle. The importance of the nuclei in question has been discussed in many publications; sensitivity studies have led to data accuracy requirements. Therefore this section summarizes the main aspects of the formation and importance of MINACs in relation to nuclear data, to give an adequate background for the subsequent discus- sion. 2.1 General_______--- Importance -______--------__--------- of Minor Actinides The following summary briefly describes the main reactor physics aspects of the MINAC isotopes in the various stages of the nuclear fuel cycle (see e.g. /4/j. 92120003 . General Importance of Minor Actinides (MINACs Fuel Cycle stage Reactor Physics Aspects Important MINACs a) In-Pile: Reactivity (F.> 10m3) U234, U236 cycle length, (breeding, Np237, Pu238(FBR) conversion) Pu242, Am241(R)+) Am243(R)+) With the exception of U236, total reactivity effects are small, partly due to compensation. b) Shut-Down: Reactivity margin, U234, U236 subcriticality, Np237, Pu242 0 (heat is mainly Am241 (R) produced by FP) An.243 (R) c) Transportation and Subcriticality, U236, Np237 interim storage: radiation (n + y) Am241, Cm242 Cm244 d) Reprocessing: Radiolysis Pn238 Separation Cm244 Subcriticality in Np237 dissolver (low burn-up) (activity : FP) a e) Refabrication: n + y radiation Pu236 U232 TL208 Pu-aerosols Pu238 Am241 f) Waste: n + y radiation during vitrification Pu losses long-term hazard Am241 Am243 Cm244 +I R = Pu recycling, i.e. important in recycling strategies. FP = Fission products r . 2.2 ------------The Inventory ------------------------of Minor Actinide Isotopes----------------- in Thermal and --_______--------___---------------------Fast Reactors of 1250 MWe at Fuel Discharge - As a measure of practical. importance of minor actinides the concentration of these isotopes in actual reactors is presented in this section. Present-day commercial PWRplants, like the RWU-Biblis reactor, have an electrical output of about 1250 MWe. For this reactor type, the inventory of MINACS is calculated for two burn-up values: for 33 GWd/t and for the extended burn-up of 40 GWd/t (even higher burn-up values are envisaged, but a natural limit lies in the range between 45 - 50 GWd/t, if zircaloy is used as clad material in these reactors). The inventory is calculated at fuel discharge /5, 6/. Major changes in MINAC-concentrations after dis- charge occur fbr the following isotopes: Pu236 (2.9~ half-life) +U232, ~~241 (14.9y) *Am241 and Cm242 (163d) -f Pu238; the corresponding concen- trations of these isotopes at any further stage of the fuel cycle (e.g. at reprocessing) can easily be evaluated. Furthermore, the inventory of the mixed oxide (MOX) fuel part in a mixture of a 70% U- and 30% MOX-fuelled PWR, as foreseen in Pa-recycling /7/ is given; for this type of reactor the burn-up was also calculated to 33 GWd/t to be compared to the results from a usual fuel loading. It should be mentioned that in the Pu-recycling case the MINAC concentration is reduced in the dissolution process of a reprocessing plant, due to common reprocessing of MOX- and uranium fuel elements. Results are collected in Tab. 2-l. It can be seen that the increase in burn-up mainly influences pUi36, Pu238 and, to a major extent, 0~244; in the case presented here, the Cm244 content is roughly doubled in a MOX- compared to an initial uranium loading. In Tab. 2-2, the MINAC concentrations in a Fast Reactor of 1250 MWe (SuperphGnix type) are given at fuel discharge; the core burn-up is 63 GWd/t, the mixed core and axial blanket burn-up in a common reprocessing strategy is 41 MWd/t. For comparison, the inventories are calculated for a start-up loading with PWR-plutonium, and for an equilibrium cycle /8/. As is expected, the MINAC concentrations are all larger for the start-up case with PWR-plutonium fuel; of main importance is the increase in Pu236, 92120005 . Pu7.38 and the curia isotopes. The reason for the reduced concentration in the equilibrium case is the common reprocessing of core and axial blanket material, changing mainly the Pu-composition due to the admixture of almost pure Pu'239 (i.e. practically no higher plutonia or MINACs) from the blankets. 2.3 Formation______-----------__--- Routes of Important----------------L------------~~~~--~~--- Actinides Related Cross-Sections _--------------------and their Accuracy Requests----- From the previous discussion the most important MINAC isotopes in the U-Pu fuel cycle are: U236, Np237, Pu236, Pu238, A&41, Am243, Cm242 (decreased importance due to short half-life) and Cm244. U236 has a high neutron cap- ture cross-section and influences strongly the reactivity balance end of cycle. Np237 is on the main route in U-fuelled reactors for the build-up of Pu236, which decays finally to T1208 with a very strong y-radiation. The strong a-decay of Pu238 is mainly responsible for radiolysis in repro- cessing, in addition it forms a relatively intensive source for neutron emission.